A satellite navigation system is a satellite based radio-navigation system in which a plurality of satellites orbit the earth and broadcast information that can be used by receivers on the earth to determine a position relative to the satellites. Example satellite navigation systems include the global positioning system (GPS), the GLONASS system, the Beidou navigation system, and the Galileo positioning system.
The GPS includes 24 or more satellites orbiting the earth at a height of 20,000 km in 6 orbital planes. These satellites broadcast ranging codes and navigation data on one or more of three different frequencies known as L1 (1,575.42 MHz), L2 (1,227.6 MHz), and L5 (1,176.45 MHz). A GPS satellite transmits navigation data containing various parameters that provide information about the satellite's position and health. Each GPS satellite transmits a unique ranging code that can be used by a GPS receiver to compute a distance from the GPS receiver to the satellite. This distance is referred to as a pseudorange, because it is uncorrected for the receiver's local clock error.
If there is no other information available to a GPS receiver, the receiver should make pseudorange measurements to 4 or more satellites in order to accurately compute the position, velocity, and time for the GPS receiver. The pseudorange measurements made by GPS receivers can be prone to several error sources like the ionospheric propagation delay, tropospheric propagation delay, satellite clock error, user clock error, multipath error, and other unexpected error sources. The unexpected error sources can include a sudden sizeable error referred to as a “step error” or a gradually increasing error referred to as a “ramp error”. Such step or ramp errors typically occur rarely (e.g., a probability of about 0.0001 per flight hour), but can lead to errors in the computed position, velocity, and time.
Aviation-grade GPS receivers have been required to protect against the existence of a single faulted measurement (e.g., to detect both step and ramp errors) amongst all available measurements. Many conventional GPS receivers are well equipped to deal with the existence of a single faulted measurement. The GPS receivers typically exclude the faulted measurement from use in a navigation solution whenever possible. When it is not possible to exclude the faulted measurement, the GPS receivers can provide a warning to indicate that the output position may be corrupted and cannot be relied upon.